Microdisplays are becoming increasingly popular as low cost, low power consumption, yet high resolution replacements for traditional information display components such as cathode-ray tubes. Their small size allows integration into hand-held products, such as camcorders and digital still cameras, while their high resolution capabilities promote usefulness in projection applications, such as televisions and business projectors.
The usefulness of microdisplays in projection applications depends greatly on the system's ability to project a sufficiently bright image. However, certain types of microdisplays, liquid crystal microdisplays in particular, require optical elements such as polarizers or diffusers that reduce the amount of light that reaches the viewing area. Moreover, the algorithms used to operate microdisplays to produce images also may result in wasted light, as will be explained immediately below.
Field-sequential color microdisplays produce color images by dividing an image frame into color segments. An image frame is a period of time during which the information necessary to produce a single image is displayed on the display device. A color segment is a portion of a frame during which the image information for a single color is displayed while the display is illuminated with that single color of light. Field-sequential color displays can be contrasted with non-sequential color systems, which usually combine three different color images simultaneously. By using the three primary colors, red, green and blue (RGB), in sequence, a field-sequential color display is capable of producing images from a palette of many colors. The size of the color palette is further increased by adding grayscale.
Grayscale refers to “shading” colors—varying the amount of each primary color included in the image—thus increasing the number of combined colors the system is capable of producing. Field-sequential color systems produce grayscale in one of several ways. One method is to vary the intensity of light either reflected by or transmitted through the device. A second method is to vary the duration of time that the light is reflected or transmitted. Methods for producing grayscale in microdisplays are well known. For example, U.S. Pat. No. 5,748,164, issued May 5, 1998, entitled Active Matrix Liquid Crystal Image Generator, describes various methods for producing gray-scale images in field-sequential color microdisplays, which patent is incorporated herein by reference in its entirety.
In prior art methods of producing gray-scale images in field-sequential color systems, light may be underutilized. For instance, because image information cannot be written to the entire display as fast as the light source can be switched to a different color, color transitions can result in image artifacts unless ameliorative steps are taken. One common ameliorative step is to make the display dark during the time that the light source is switched to a different color. Unfortunately, any light emitted while the display is dark is essentially wasted.
Light may also be wasted even if image information can be written to the entire display as fast as, or faster than, the light source can be switched to a different color. Most methods of switching a light source between colors do so in a finite amount of time, producing intermediate states of illumination that are either of a different color or a different intensity from the desired pure colors before and after the transition. For example, a color wheel that transitions between red and green will produce yellow light during the transition. A liquid crystal color switch may avoid such intermediate colors but will then produce intermediate light of varying intensity. In both cases the intermediate light is generally considered unusable directly by the display element in a field sequential color system designed to operate with primary colors of a single intensity. Again, a common ameliorative step is to make the display dark during the transition.
Light is also wasted in liquid crystal display systems that use a compensator cell while DC balancing the liquid crystal material. Compensators are more fully explained in U.S. Pat. No. 6,075,577, issued Jun. 13, 2000, entitled Continuously Viewable, DC Field-Balanced, Reflective Ferroelectric Liquid Crystal Image Generator, which patent is incorporated herein by reference in its entirety. DC balancing is desirable to prevent image sticking or image retention. It is achieved by inverting the electrical polarity sense of the pixel drive voltage. In the case of liquid crystal displays incorporating polarity responsive liquid crystal materials such as ferroelectric liquid crystals, reversing the electrical polarity sense of the pixel drive inverts the appearance of the image. In such cases, a compensator may be used to allow the inverted image to be displayed without affecting the appearance of the image at the image area. However, as with changing the color of the light source, the compensator can be switched much faster than data can be written over the entire display. Thus, to avoid image artifacts, one prior art solution is to make the display dark during a compensator transition.
A number of additional factors similar to those briefly discussed above also result in inefficient use of the available light in field-sequential color display systems. It is against this backdrop and a desire to solve the problems of the prior art, including a desire to increase the brightness of field-sequential color displays, that the present invention has been developed.